1. Field of the Invention
This invention relates to a process for solvent refining of petroleum.
2. Description of the Prior Art
Numerous solvents have been used to separate aromatic and paraffinic constituents of hydrocarbon streams. Such separation is desirable because the aromatic constituents and gasoline fractions contribute toward high octane numbers while the straight chain paraffinic constituents greatly reduce octane numbers. Furthermore, pure benzene, toluene and xylene are important raw materials in the petrochemical industry. Paraffinic hydrocarbon streams are used as constituents of heating oils and jet fuels.
The solvents used in said extraction processes must meet critical requirements of (1) being only partially miscible with the hydrocarbon feed streams, (2) having a high hydrocarbon extraction capacity and (3) having a high selectivity for aromatic hydrocarbons. High capacity is important since this reduces the amount of solvent necessary to achieve the desired separation and thereby reduces the size of the vessels and associated pumps needed for recirculation. Selectivity (.beta.) is also important. This is defined as ##EQU1##
Solvents having a high selectivity require fewer stages of extraction to achieve a given degree of separation. This in effect reduces the height of the extraction vessel. Generally speaking, the higher the capacity of the solvent, the lower the selectivity. Therefore some compromise is generally made between these properties in selecting a suitable solvent for the separation desired.
Other desirable characteristics of a good extraction solvent include the following: (4) low molecular weight, (5) noncorrosiveness to the equipment at operating conditions, (6) complete stability under operating conditions, (7) easy recoverability from the hydrocarbon products and (8) low cost.
No extraction solvent has yet been discovered which satisfies all of the foregoing requirements for the separation of paraffinics and aromatics in hydrocarbon streams. A number of solvents have been used including sulfolane, N-formyl morpholine, various glycols and mixtures thereof, etc. The use of most of these solvents requires, subsequent to the extraction steps, that either the solvents or the hydrocarbon is removed from the aromatic-rich solvent phase by distillation. This requires a large amount of heat which is a major cost item in the extraction process.
The use of low molecular weight aliphatic alcohols as extraction solvents is well known in the art. For example, U.S. Pat. Nos. 1,781,421 and 1,783,203 disclose the use of substantially anhydrous alcohols for this purpose. U.S. Pat. Nos. 2,770,663 and 2,727,848 disclose that water and aliphatic alcohols can be admixed for use in solvent extraction. The specific aliphatic alcohols illustrated however are glycols rather than monohydric alcohols.
U.S. Pat. No. 3,119,767 discloses a method of separating aromatic and paraffinic constituents in hydrocarbon mixtures by use of mixtures of methanol and water or ethanol and water. The volume of water in these mixtures however is limited to 5%. As indicated therein, extraction with the use of these solvents requires subsequent distillation of the aromatic-rich solvent phase in order to recover the solvent.
U.S. Pat. No. 3,985,644 discloses separating naphtha into aromatic and paraffin-rich fractions by solvent extraction with a methanol-water mixture. The solvent is separated from the raffinate phase by lowering of the temperature of the mixture rather than by using additional water as an antisolvent.
U.S. Pat. No. 2,261,780 discloses separating naphtha into aromatic and paraffin-rich fractions by solvent extraction with a phenol-water mixture. The solvent is separated from the extract phase by the addition of a precipitant to the solvent-extract mixture. This causes the precipitation of a large portion of the oil from the primary solvent. The primary solvent mixture is then scrubbed with a secondary solvent in order to remove the remaining oil from the solvent.
U.S. Pat. No. 2,646,387 suggests a solvent recovery technique which employs liquid carbon dioxide at non-critical conditions to separate the solvent from the extract and naphthenate fractions.
There is a great incentive in seeking an extraction solvent system which would be lower in cost than those heretofore used commercially. Furthermore, there is additional incentive in seeking an extraction solvent system in which the solvents can be separated from the aromatic-solvent phase without the necessity of distillation. This invention provides a solvent extraction system meeting these requirements. The use of said systems results in a process for separation of aromatic and paraffinic hydrocarbons which is economically attractive and the solvent is quite inexpensive and can be separated from the aromatic-rich phase by simple addition of water to decrease the solubility of the aromatic-rich extracted product in the methanol solvent.